Early Arthropods: The fossil record
An early segmented animal was the trilobite. These animals had a bony armour composed of lime and a horny substance called chitin. The armour was not expandable and therefore shed periodically. Many of these shed exoskeletons have been preserved as fossils. Where the entire animal is preserved you can observe the jointed legs that are attached to each segment of the body, the feathery gill next to each leg, two feelers at the front of the head, the gut running the length of the body, and even muscle fibres along the back which enabled the animal to roll itself into a ball. Comparatively high resolution eyes composed of mosaics of separate cells and a crystalline calcite lens. The very thick lens of some trilobites may have reflected their colonization of deeper water where light is considerably reduced. However, the optimal properties of the calcite lens operating in water would not have permitted a fine focus. This shortcoming was compensated by the evolution of the two-part lens with a waved surface at the junction of the two lens elements.